In a large bowl, combine burger ingredients and mix to combine, being careful not to over mix.
Shape into 6 patties, each 1/2-inch ( about 1 cm) thick.
Reduce heat to medium and place burgers on lightly greased grill, cover and cook for about 8 minutes on each side or until an instant-read thermometer inserted sideways into the centre reads 165°F (74°C).
Meanwhile, in a skillet melt butter over medium heat and sauté mushrooms until golden.
Stir in capers, lemon zest and juice and sauté for 5 minutes.
Place burger patties on buns.
Top with lettuce, tomato, and mushroom mixture.
Serve with tzatziki.
Nutrients per serving: 470 calories, 23 g fat, 34 g carbohydrates, 3 g fibre